1. Field of the Invention
This invention generally relates to electric insulation materials. More particularly, this invention is directed to an extruded groundwall insulation material for a stator bar of an electric machine and a process for applying the insulation material to the stator bar.
2. Description of the Related Art
FIG. 1 represents an end portion of a stator (armature) bar 10 of the type installed in dynamoelectric machines, such as a generator used in power generation of high-voltage alternating current. The stator bar 10 can be manufactured to have essentially any length, shape and cross section appropriate for a given generator design, voltage, and power. For most high-voltage applications, the stator bar 10 will not have a simple linear shape, but instead will have a complex shape with bends and turns.
As shown, the stator bar 10 is composed of a number of conducting copper strands 12 that are insulated from each other by strand insulation 13. The strands 12 are arranged to form two tiers that are separated by a strand separator 14, all of which may together be termed a bare bar. Surrounding the tiers is a stator bar (groundwall) insulation 15 formed by multiple wrappings of a mica paper tape 16. Typically multiple layers of tape are tightly wrapped around the conductor, usually overlapping by one-half the width of the tape, or half-lapped. The groundwall insulation 15 serves to insulate the stator bar 10 from the stator in which it is installed.
Groundwall insulation of the type shown in FIG. 1 is widely used in the power generation industry. The mica paper tape 16 is a prepreg composed of a mica paper typically backed by a single woven backing or a pair of backings. A resin composition permeates the mica paper and bonds each backing to the mica paper, thereby forming the prepreg tape. Examples of this type of groundwall insulation include commonly-assigned U.S. Pat. No. 3,563,850 to Stackhouse et al., U.S. Pat. No. 5,618,891 to Markovitz, U.S. Pat. No. 6,043,582 to Markovitz et al., and U.S. Pat. No. 6,359,232 to Markovitz et al. After being wrapped with a sacrificial release film to protect the tape and prevent contamination, the stator bar 10 are placed in an autoclave for vacuum heat treatment and subsequent curing of its tape 16. Vacuum heat treatment is carried out to remove air, moisture and any solvent or volatile compound present in the resin binder of the tape 16 while curing under pressure serves to consolidate the tape insulation, such that the resin binder bonds the mica paper and each of its backings together to form a void-free solid insulation. Removal of air, moisture, solvents and volatile compounds from the binder is necessary to prevent formation of voids in the cured insulation that would otherwise adversely affect the quality of the insulation and induce premature insulation failure due to breakdown under electrical stress. The latter characteristic of insulation is termed “voltage endurance,” and is normally due to erosion by electrical discharge and electrochemical attack.
It can be appreciated that groundwall insulation of the type described above is labor intensive and incurs significant process costs. Furthermore, if not properly controlled, the taping process can lead to the presence of voids, resulting in reduced performance reliability. Therefore, improved groundwall insulation and processes have been investigated. For example, extruded groundwall insulation has been proposed, examples of which include commonly-assigned U.S. Pat. No. 5,650,031 to Bolon et al. and U.S. Pat. No. 5,710,475 to Irwin et al. In these approaches, the bare stator bar is passed through an extrusion die, which deposits the groundwall insulation in-situ along the entire length of the bar. The technical challenges associated with extruded groundwall insulation, including the difficulty of passing stator bars with complex shapes through a die, have been significant, such that stator bars equipped with extruded insulation are not currently in production. Accordingly, there remains a demand for groundwall insulation that overcomes the shortcomings of groundwall insulation formed of multiple wrappings of mica paper tape.